Measurement devices for measuring a property of a fluid flowing through a pipe are commonly used in various branches of industry, for example in the mining or mineral processing industry or in the chemical industry. The measurement data obtained by these devices is e.g. used to control complex industrial processes.
Such measurement devices include e.g. flow meters using different measurement principles, e.g. differential pressure flow meters, electromagnetic flow meters, ultrasonic flow meters, coriolis or vortex flow meters.
In order to be able to measure a property of a flowing fluid, the measurement devices comprise at least one measurement tube, to be inserted in and connected to a pipe, such that at least part of the fluid flowing through the pipe flows through the measurement tube during measurement. Measurement tubes normally consist of metal, e.g. stainless steel and are quite often equipped with a liner, covering the inside of the measurement tube.
Electromagnetic flow meters make use of Faraday's law of induction which states that a voltage is induced in a conductor moving in a magnetic field. In electromagnetic flow meters the fluid flowing through the measurement tube corresponds to the moving conductor. The induced voltage is proportional to the flow velocity and is detected by measuring electrodes. Here a volume flow flowing through the measurement tube is determined based on the measured induced voltage and the inner diameter of the measurement tube. The constant magnetic field is e.g. generated by a switched direct current of alternating polarity. Liners of electromagnetic flow meters consist of electrically insulating materials providing electrical insulation between the measuring electrodes and the measurement tube.
Liners of measurement tubes are directly exposed to the fluid. Depending on the application and/or the properties of the fluid deposits of the fluid may build up on the inside of the liner. In addition the liner may by subject to abrasion, e.g. due to mechanical and/or chemical properties of the fluid, reducing the thickness of the liner.
Deposits as well as abrasion will alter the measurement properties of the device. Thus there is a need in industry to detect either of them at a very early stage, in order to enable the operator to take appropriate countermeasures, long before the device fails or measurement errors exceed a maximum permissible level.
With respect to electromagnetic flow meters abrasion is more critical than accretion, because a decreasing thickness of the liner will directly affect the required insulation between the measurement electrodes and the measurement tube.
U.S. published application, 2013/0031973 A1 describes a method of detecting accretion and/or abrasion of a measurement tube of a Coriolis flow meter, which is in direct contact to the fluid flowing through it. To this extend the flow meter is equipped with two temperature sensors. The first temperature sensor is mounted on an outside wall of the measurement tube, subjected to accretion and/or abrasion. The second temperature sensor is preferably mounted on an outside wall of a pipe segment connected to the measurement tube, in a region, where accretion and/or abrasion is unlikely to occur. Both temperature sensors are applied to measure the temperature prevailing at their location as a function of time. Accretion and/or abrasion is then detected based on a time dependency of the relation of the two temperatures measured as function of time.
In addition Published International Application, WO 2009/134268 A1 describes a method of detecting accretion of a measurement tube of a Coriolis flow meter, which is in direct contact to the fluid flowing through it. Here accretion is detected based on a temperature gradient between spaced apart locations along the flow meter. To this extend temperature sensors are mounted on spaced apart locations on the outside of parts of the flow meter which are exposed to the fluid on their inside, e.g. on an outside wall of a tube inlet and a tube outlet connected to the measurement tube, or on the outside walls of two separate measurement tubes.
Whereas measurement tubes described in the prior art generally consist of metals having a very high thermal conductivity, thermal conductivity of liners, in particular of electrically insulating liners used in electromagnetic flow meters, may be fairly low. Thus even if it may be possible to apply the methods described in US 2013/0031973 A1 or in WO 2009/134268 A1 to measurement devices comprising measurement tubes, which are equipped with liners, the liner will have a noticeable effect on the measured temperatures and the time dependencies of the measured temperatures, which will have to be taken into account.